1. Field of the Invention
The present invention relates generally to fluoropolymeric substrates with metallized surfaces and methods for producing such metallized surfaces.
2. Discussion of the Background
Fluoropolymers such as poly(tetrafluoroethylene) (PTFE) and Telfon.RTM. are of considerable technological importance, because their low surface energy and stable C--F bonds provide surfaces that are inert to most solvents and chemicals and that prevent the adhesion of most chemical and biological materials (P. Burggraf, Semicond. Int., vol. 11 (no. 8), p. 55 (1988)). The low dielectric constants of fluoropolymers make them particularly attractive as dielectric layers for microelectronic applications (L. M. Siperko et al, J. Adhes. Sci. Technol., vol. 3, p. 157 (1989); H. Meyer et al, in Metallized Plastics 2: Fundamental and Applied Aspects, L. Mittal, Ed., at p. 121, Plenum, N.Y. (1991)). However, for certain applications in which it is desirable to use fluoropolymers as a substrate, relatively few chemical pathways exist for the stable attachment of materials to the fluorinated surfaces.
Approaches for promoting adhesive bonding of various materials, including metals, to fluoropolymer surfaces typically use harsh chemical reagents (highly reducing alkalies, such as sodium naphthalide) or require complex sputtering or ion beam bombardment process (L. M. Siparko et al, J. Adhes. Sci. Technol., vol. 3, p. 157 (1989); H. Meyer et al, in Metallized Plastics 2: Fundamental and Applied Aspects, L. Mittal, Ed., at: p 121, Plenum, N.Y. (1991)). A recently reported process (R. R. Rye et al, J. Electrochem. Soc., vol. 139, L60 (1992)) involves cross-linking of PTFE with x-rays followed by chemical etching and then vapor deposition of Cu by decomposition of an organocopper reagent. This methods are often difficult to use, may be environmentally problematic, and can adversely affect the chemical and morphological characteristics of the surface.
U.S. Pat. No. 4,548,867 (Ueno et al) discloses a fluorine-containing synthetic resin having improved surface properties as evidenced by increased wettability with water, printability and susceptibility to adhesive bonding. The fluoropolymer is exposed to a low temperature plasma comprising an organic nitrogen-containing gas. Instead of modifying the atomic composition of the fluoropolymer starting material, Ueno et al form a ti "layer" of a nitrogen-containing wettable material thereto. Consequently, the adherence of such an overcoating tends to alter the microstructural morphology of the original polymer, especially with respect to pore size. This coating also alters desirable surface properties exhibited by the original fluorinated material.
In some early work, it has been found that exposure of polyolefins and perfluorinated polymers to low power radio frequency electroless discharges in inert gas atmospheres produced favorable results over wet chemical methods. Their improvement in the bondability of surfaces was limited and attributed to the formation of a highly cross-linked surface layer. Studies of Hollahan et al, J. Polym. Sci., vol. 13, p. 807 (1969) aimed at rendering polymer surfaces biocompatible included the interaction of PTFE with plasmas excited in ammonia and nitrogen/hydrogen mixtures, the goal being the introduction of amino groups into the polymer surface. However, the long exposure times and high powers employed provided only limited results, and further, are thought to have produced significant changes not only in the surface chemistry, but also the native bulk properties. The morphology of the surface was also severely affected.
In another ESCA study entitled "ESCA Study of Polymer Surfaces Treated by Plasma," Yasuda et al, J. Polym. Sci., Polym. Chem. Ed., vol. 15, p. 991 (1977) the effects of discharges in argon and nitrogen on surface chemistry were considered on a range of polymers. PTFE was found to be particularly susceptible to defluorination and the introduction of oxygen and nitrogen moieties into the surface.
It has recently been shown the fluoropolymers can be functionalized by chemisorption of organosilane reagents to plasma-treated fluoropolymer surfaces (T. G. Vargo et al, J. Polym. Sci. Polym. Chem. Ed., vol. 29, p. 555 (1991) D. J. Hook et al, Langmuir, vol. 7, p. 142 (1991); F. V. Bright et al, Anal. Chim. Acta, vol. 262, p. 323 (1992); T. G. Vargo et al, Langmuir, vol. 8, p. 130 (1992); J. P. Ranieri et al, J. Biomed. Mater. Res., vol. 27, p. 917 (1993); and U.S. Pat. No. 5,266,309). Radio-frequency glow discharge (RFGD) treatment of the fluoropolymer surface using a novel gas-liquid mixture (T. G. Vargo et al, J. Polym. Sci. Polym. Chem. Ed., vol. 29, p. 555 (1991)) partially defluorinates the surface with simultaneous addition of hydroxyl functionalities. An important aspect of this plasma treatment is that the surface is modified without inducing significant roughening. The hydroxylated surface exhibits a reactivity similar to that of Si--OH groups on silicon oxide surfaces and can be reacted with organosilane reagents to covalently immobilize various desired functionalities on the fluoropolymer surface (D. J. Hook et al, Langmuir, vol. 7, p. 142 (1991); F. V. Bright et al, Anal. Chim. Acta, vol. 262, p. 323 (1992); T. G. Vargo et al, Langmuir, vol. 8, p. 130 (1992); J. P. Ranieri et al, J. Biomed. Mater. Res., vol. 27, p. 917 (1993)). It has also been shown that use of a mechanical mask can restrict plasma treatment to particular regions of the surface; subsequent attachment of the organosilane occurs only in the areas exposed to the plasma. Such patterned aminoalkylsilane-fluoropolymer surfaces have been successfully used as chemical templates for the selective attachment and growth of neurons (T. G. Vargo et al, Langmuir, vol. 8, p. 130 (1992); J. P. Ranieri et al, J. Biomed. Mater. Res., vol. 27, p. 917 (1993)).
It has also been shown that selective, adhesive metallization of a wide range of nonfluorinated substrates to submicrometer resolution can be accomplished by electroless deposition (J. M. Calvert et al, in Polymers for Microelectronics, C. G. Wilson et al, Eds., at p. 210, ACS Symposium Series vol. 537, American Chemical Society Press, Washington, D.C. (1993); J. M. Calvert et al, Proc. Soc. Photo.-Opt. Instrum. Eng., vol. 1924, p. 30 (1993); J. M. Calvert in Organic Thin Films and Surfaces, vol. 1, A. Ullman, Ed. Academic Press, Boston, in press; J. M. Calvert et al, J. Vac. Sci. Technol., vol. B9, p. 3447 (1991); J. M. Calvert et al, Solid State Technol., vol. 34 (no. 10), p. 77 (1991); C. S. Dulcey et al, Proc. Soc. Photo.-Opt. Instrum. Eng., vol. 1925, p. 657 (1993)). Surfaces functionalized with self-assembled monolayer (SAM) films of ligand-bearing organosilanes covalently bind a Pd catalyst (W. J. Dressick et al, Chem. Mater., vol. 5, p. 148 (1993); W. J. Dressick et al, J. Electrochem. Soc., in press) from aqueous solution and are then metallized by immersion in an aqueous electroless deposition bath.
However, there is no report of a convenient method for providing a fluoropolymer substrate with a metallized surface. In addition, there is no report of a convenient method for preparing a fluoropolymer substrate with a metallized surface which exhibits good adhesion to the fluoropolymer substrate. Accordingly, there remains a need for such a method. There also remains a need for fluoropolymer substrates having a metallized surface produced by such a method.